In general, an electric motor car is configured to receive electric power from an overhead wire with a current collector, drive a motor with a power converter such as an inverter apparatus using the received power, and travel. When a car is braked, a so-called regenerative brake that regeneratively drives the motor to obtain braking force is used. Regenerative power generated at this point is supplied to loads of other cars present near the own car via the overhead wire, a third rail, or the like and consumed in the loads.
However, in the early morning or at night or in a quiet railroad section with a small number of trains in service, in some case, no other cars are present near the own car and the regenerative power generated by the own car is not sufficiently consumed. When the regenerative power of the own car is larger than electric power consumed by other cars, an overhead wire voltage rises. It is likely that various apparatuses connected to the overhead wire are damaged.
Therefore, the inverter apparatus mounted on the electric motor car includes a voltage detector that detects the overhead wire voltage or the like (the overhead wire voltage or a filter capacitor voltage, for example, on an input side of the inverter apparatus equivalent to the overhead wire voltage). When the overhead wire voltage or the like rises and exceeds a predetermined value, the inverter apparatus performs control for suppressing regenerative braking force to suppress the generation of the regenerative power and operates to prevent the overhead wire voltage or the like from rising to a specified value or more. At this point, braking force of the car in which the regenerative braking force is suppressed and insufficient is supplemented by a mechanical brake configured to press a brake shoe against a wheel or a brake disk. Brake energy (kinetic energy) of the car changes to heat and is radiated to the atmosphere. In such a case, there is a problem in that the brake energy that should originally have been able to be used in the loads of the other cars is wasted and wear of the brake shoe of the mechanical brake occurs.
In recent years, performance of power storage devices such as a secondary battery and an electric double layer capacitor is improved. Therefore, a power storage system is developed in which the power storage devices are mounted on an electric motor car and excess regenerative power, for example, during braking without a sufficient regenerative load is stored in the power storage devices and reused during power running.
This power storage system includes a converter apparatus that performs DC-DC conversion and a power storage apparatus including a power storage device such as a secondary battery or an electric double layer capacitor. In general, a method of charging and discharging the power storage device is configured to control charging and discharging currents to the power storage apparatus via the converter apparatus.
As an example of such a power storage system, for example, a related art disclosed in Patent Literature 1 is a technology for mounting an electric double layer capacitor on a car and, making use of a phenomenon in which an overhead wire voltage or a filter capacitor voltage rises during regenerative braking when no other trains as regenerative loads are present near the own car, when a detection value of the filter capacitor voltage exceeds a predetermined value, generating an absorption current command to a power storage device according to an excess amount and performing control of a converter apparatus to collect and store excess regenerative power during an overhead wire voltage rise in the electric double layer capacitor, and discharging the regenerative power during the next power running to realize reuse of regenerative energy.